The invention relates to an electromagnetic detection system which, in operation, in a detection zone, by means of at least one transmission antenna coil, generates a swept-frequency interrogation field capable of being at least partly disturbed by a responder comprising a tuned circuit, if such responder is present in said detection zone, there being provided detection means coupled with at least one detection antenna coil for detecting such disturbance.
Such systems are already known in various embodiments. In general two types of field disturbance detection systems may be distinguished. Systems of the first type are based on absorption of interrogation field energy by the tuned circuit of the responder. Absorption takes place selectively, i.e. at a pre-determined frequency or frequency band because the responder comprises a tuned circuit. Owing to the selective absorption the energy content of the transmission circuit is modulated, which modulation can be detected by means of an envelope detector, which is connected to the transmission antenna coil and which may be a simple diode. This envelope detector then issues a pulse in the form of the resonance curve of the tuned circuit of the responder. This form is known and so the detected pulse can be compared with the known form. Thus, in this case the detection antenna coil is (one of) the transmission detection coil(s).
As an alternative in a system of the absorption type a separate detection antenna coil (or receiver antenna coil) for monitoring the energy content of the interrogation field may be used. Selective absorption of energy by the tuned circuit of the responder results in a pulse-shaped disturbance of the interrogation field and thus in a pulse-shaped variation of the output signal of the detection antenna coil. This pulse-shaped variation can again be detected by suitable detection means coupled to the detection antenna coil.
In systems of the second type (transmission systems) one or more separate receiver coils are used to detect signals retransmitted by the responder. Preferably the receiver antenna coils are positioned in such manner, that they do not directly detect the interrogation field, but only detect signals retransmitted by a responder, which generally has an orientation differing from the orientation of the transmitter antenna coil(s). In such a system transmission of energy occurs from the transmitter antenna coil(s) to the responder and from the responder to the receiver antenna coil(s) (detection antenna coil(s)) when the interrogation field frequency equals the resonance frequency of the tuned circuit of the responder. Again the presence of a responder may be detected by detection means coupled to the receiver (detection) antenna coil(s).
One disadvantage of the known system(s) is that other high-frequency signals not coming from a responder associated with the system can be detected by the detection antenna coil(s) and may cause the generation of a pulse at the output of the envelope detector. These signals may have frequencies located outside the sweep of the swept interrogation frequency or within this range.
Such signals are respectively called out-band signals and in-band signals.